A plant for transmitting electric power through HVDC includes two converter stations interconnected by a monopolar direct voltage network and each having an alternating voltage side for feeding electric power from one of said alternating voltage sides to the other. Each station has a line commutated converter. The plant is upgraded by making the direct voltage network bipolar, providing each station with a Voltage Source Converter and providing two switches for both line commutated converters or both Voltage Source Converters and a device configured to control said four switches.
Legal claims defining the scope of protection, as filed with the USPTO.
1. A method of upgrading a plant for transmitting electric power through High Voltage Direct Current (HVDC), said plant comprising two converter stations interconnected by a pole conductor of a monopolar direct voltage network and each having an alternating voltage side for feeding electric power from one of said alternating voltage sides to the other, each station having a line commutated Current Source Converter having at least one phase leg with one end connected to said pole conductor of the direct voltage network and the other end having an earth connection in the form of a connection to an earth electrode or a return current conductor, the method comprising: interconnecting said stations by a further pole conductor such that said direct voltage network is made bipolar, and providing each station with a Voltage Source Converter having at least one phase leg, one end of which is connected to said further pole conductor and the other end to said earth connection and by that to said Current Source Converter, the method further comprising: providing two switches for both Voltage Source Converters of the plant, and providing means for controlling said four switches so as to enable to change, when the feeding direction of power transmitted between said stations is to be changed, which end of a said phase leg of each converter provided with said switches is connected to one of said pole conductors and which end is connected to said earth connection and by that to the other converter of the station so as to maintain the direction of the current flowing in each said pole conductor upon a said change of the feeding direction of the power transmitted between said stations.
2. A method according to claim 1 , wherein each said station is provided with said Voltage Source Converter, each said phase leg of which comprises a series connection of switching cells, each switching cell having on one hand at least two semiconductor assemblies connected in series and having each a semiconductor device of turn-off type and a free-wheeling diode connected in parallel therewith and on the other at least one energy storing capacitor, each said switching cell being configured to obtain two switching states by control of said semiconductor devices of each switching cell, namely a first switching state and a second switching state, in which the voltage across said at least one energy storing capacitor and a zero voltage, respectively, is applied across the terminals of the switching cell.
3. A method according to claim 1 , wherein each said station is provided with said Voltage Source Converter, each said phase leg of which comprises a series connection of current valves, each said current valve having on one hand a semiconductor device of turn-off type and a free-wheeling diode connected in anti-parallel therewith.
4. A method according to claim 1 , wherein each station is provided with two said switches in the form of high voltage breakers.
5. A method according to claim 4 , wherein each station is provided with said switches in the form of SF6-breakers.
6. A method according to claim 1 , wherein the further conductor is a cable.
7. A plant for transmitting electric power through High Voltage Direct Current (HVDC) comprising two converter stations interconnected by a bipolar direct voltage network and each having an alternating voltage side for feeding electric power from one of said alternating voltage sides to the other, each station comprising: a line commutated Current Source Converter having at least one phase leg with one end connected to a first pole conductor of the direct voltage network and the other end having an earth connection in the form of a connection to an earth electrode or a return current conductor, and a Voltage Source Converter having at least one phase leg, one end of which is connected to a second pole conductor of the bipolar direct voltage network and the other end to said earth connection and by that to said Current Source Converter, the plant further comprising: two switches for both Voltage Source Converters, and means for controlling said four switches so as to enable to change, when the feeding direction of power transmitted between said stations is to be changed, which end of a said phase leg of each converter provided with said switches is connected to one of said pole conductors and which end is connected to said earth connection and by that to the other converter of the station so as to maintain the direction of the current flowing in each said pole conductor upon said change of the feeding direction.
8. A plant according to claim 7 , wherein said two switches of each station are high voltage breakers.
9. A plant according to claim 7 , wherein said converters are configured to have a direct voltage across said two pole conductors of the bi-polar direct voltage network being between 1 kV and 1200 kV or between 10 kV and 1200 kV or between 100 kV and 1200 kV.
10. A plant according to claim 7 , wherein it is configured to conduct a direct current of 200 A to 10 kA or 1 kA to 7 kA through said direct voltage network from one converter station to the other.
11. A plant according to claim 7 , wherein the further conductor is a cable.
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December 17, 2008
October 21, 2014
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